High-temperature corrosion-resistant austenitic steel



United States Patent 5 Claims. 231. 75-123 This invention relates to corrosion-resistant austenitic steels possessing improved mechanical and chemical resistance properties at high temperatures. The invention is more specially though not exclusively directed to such steels having improvedcorrosion-resistant characteristics in the presence of lead oxide and which will consequently be highly suitable for use in the construction of certain parts of internal combustion engines, including especially the exhaust valves thereof, which are liable to be exposed to the combustion products of lead tetraethyl as present in high-grade present-day fuels.

In recent years there have been developed grades of austenitic stainless steel having good corrosion-resistant properties at high temperatures, and characterized in that they include, in addition to iron, carbon, a major proportion of chromium, some manganese and nickel, a small proportion of nitrogen as well as silicon. A typical range of compositions for such austenitic steels may be given as follows: carbon 0.5-0.7%; chromium -22%; manganese 712%; nickel 3-5 nitrogen 0.5%; and silicon 0.45%.

While it would be desirable to increase the silicon content in such a steel in order to improve its hightemperature oxidation resistance in air and also improve the characteristics of the molten metal especially as to freedom from blowholes, fineness and toughness of grain, and hence its forgeability, tests have shown that an increase in silicon content considerably lowers the chemical resistance of the resulting steel compositions in the presence of molten lead oxides at high temperatures. This is a major inconvenience in cases where the steel is used in the construction of certain parts of internal combustion engines for automobiles and aircraft, which in service are exposed to the lead oxide produced in the combustion of anti-knock fuels containing lead tetraethyl. The lead oxide is frequently present in molten form in certain parts of the engine at a temperature of 915 C., and it has been found that under these conditions the resulting corrosion resistance of the steel drops drastically with an increase of the silicon content therein. Thus, when the silicon content in a steel of the exemplary composition given above is increased from 0.19 to 0.41%, the weight loss on exposure to molten lead oxide rises from 3.53 to 15.85%. Higher percentages of silicon are accompanied by even higher percentages of weight loss in the presence of molten lead oxide.

On the other hand, it has been found that in the presence of lead oxide at a temperature below the melting point thereof, at a temperature of, for instance, 750 C., the corrosion resistance of the steel would actually be increased with an increase in silicon content beyond the range indicated above.

An increase of the silicon content also results in improving the resistance to oxidation of said steels, at high temperatures, in the air and combustion-gases free from lead-compounds, and allowing the obtention of a clean molten metal, the forgeability of which is much improved.

Objects of this invention include the provision of an improved steel composition which will exhibit excellent mechanical and chemical resistance characteristics at high 3,3l0,39 Patented Mar. 21, 1967 temperatures and especially in the presence of lead oxides whether in the solid or the liquid phase; and hence to provide improved corrosion-resistant steels highly suitable for the manufacture of various parts of internal combustion engines, such as exhaust valves, in the automotive, aircraft and related fields.

In accordance with the invention, it has been found that in austenitic steels of the type referred to, it becomes possible to increase considerably the silicon content therein with the consequent beneficial results indicated above, provided there is added to the steel composition a small but significant amount of a highly oxidizable element, specifically niobium.

Preferably, the niobium content added to the austenitic steels referred to above is between 0.1 and 1%.

The experimental work underlying this invention has brought to light the unexpected fact that the addition of as little as 0.2% or even less niobium into an austenitic steel composition of the specified type having a relatively high silicon content, brings about a marked decrease in the corrosion rate of the steel at high temperatures, in a medium of lead oxide molten at 915 C., while retaining or even improving the other requisite characteristics of the steel including its mechanical strength as well as its chemical resistance to molten lead oxide and other'corrosive and/ or oxidizing agents as may be present e.g. in the exhaust gases of an internal combustion engine, over the whole operative temperature range involved.

An improved steel composition according to the invention may comprise the following formulation by weight, in addition to iron and the usual impurities in the usually accepted ranges:

Percent Carbon 0.4-0.8 Chromium 18-23 Nickel 2.0-5.0 Manganese 7-12 Nitrogen 0.2-0.5 Silicon 0.45-0.80 Niobium 0.1-1.0

A range of compositions which has shown especially successful results and is hence preferred according to the invention is the following, again by Weight and in addition to iron and the usual impurities:

Percent Carbon 0.50-0.65 Chromium 20-22 Nickel 3.0-4.5 Manganese 8.0-11.0 Nitrogen 0.32-0.38 Silicon 5.5-0.6 Niobium 02-06 Percent Fe usual impurities about 63.40-64.60 C 0.55 Cr 21.0 Mn 9.50 Ni 4.00 N 0.35

Total weight Test. batch, No. Added Si, Added Nb, loss in molten percent percent lead oxide at 915 C percent 0. 25 0. 1. 0 0. 0. O. 8 0. (i0 0. O0 3. 2 0. 0. 20 1. 6 0. 60 0. 40 1. 2 0. 60 0. 60 1. 0

It will be noted from the above table that the test batches Nos. 1, 2 and 3 were not formulated in accordance with the teachings of the invention, whereas batches Nos. 4, 5 and 6 were. 1t will likewise be seen that of the three control samples Nos. 1, 2 and 3, samples Nos. 1 and 2 had a low silicon content (0.25%) and accordingly showed a low weight loss in molten lead oxide. As will be understood however from earlier explanat-ions, the low silicon content in these samples resulted in poor performance from other standpoints, including relatively low resistance to oxidation and poor forging characteristics, so that in spite of the apparently good results of the molten lead oxide corrosion tests shown in the table, the parts were comparatively unsatisfactory. In test sample No. 3, it was attempted to overcome these defects through a marked increase in the silicon content, herein 0.60%, though without any niobium addition. Immediately the degree of corrosion of the part in molten lead oxide was found to increase by a factor of more than three, specifically was 3.2%.

Test parts Nos. 4, 5 and 6 made in accordance with the teachings of the invention all contained the same high silicon content of 0.60% as in control par-t No. 3, and contained increasing proportions of niobium. It will be seen that the degree of corrosion of the metal in molten lead oxide decreased as a direct function of the increase in niobium content, and that in test part No. 6 containing 0.60% Nb, the corrosion was almost as low as in control sample No. 2, containing only 0.25% Si and 0.40%Nb. Even in test part No. 4 containing only 0.20% Nb, the corrosion was twice as low as in sample No. 3 containing the same amount of silicon but no niobium. At the same time all of the test samples Nos. 4 through 6 made in accordance with the invention exhibited the high allaround characteristics due to the high silicon content therein, including freedom from oxides and blowholes, grain fineness and toughness, good forgeability, and high corrosion resistance especially to lead oxide at high temperatures.

What is claimed is:

1. A corrosion-resistant composition of austenitic steel consisting essentially of, by weight, substantially 0.4- 08% carbon, 18-23% chromium, 712% manganese, 25% nickel, 0.2-05% nitrogen, OAS-0.80% silicon, and from about 0.1 to about l.0% niobium, the balance being substantially iron.

2. An austenitic steel composition consisting essentially of substantially 0.4-0.8% carbon, 1-8-23% chromium, 7l2% manganese, 25% nickel, 0.2-0.5% nitrogen, OAS-0.80% silicon, and from about 0.1 to about 1.0% niobium, all by weight, the balance being substantially iron, and characterized by its high corrosion resistance properties in the presence of both solid and molten lead oxide, at high temperatures.

3. A high temperature corrosion-resistant austenitic steel composition consisting essentially of substantially 0.500.65% carbon, 20-22% chromium, 8ll% manganese, 3.04.5% nickel, 0.32-0.38% nitrogen, 05-06% silicon, and from about 0.2 to about 0.6% niobium by weight, the balance being substantially iron.

4. Cast article for use as a part of an internal combustion engine and composed of a high-temperature corrosion-resistant austenitic steel consisting essentially of substantially 04-08% carbon, 1823% chromium, 7- 12% manganese, 25% nickel, 0.2-0.5% nitrogen, 0.45 0.80% silicon, and from about 0.1 to about 1.0% niobium, all by weight, the balance being substantially iron.

5. Exhaust valve for use in an internal combustion engine, composed of a high-temperature corrosionresistant austenitic steel consisting essentially of substantially 0.40:8% carbon, l8-23% chromium, 7-l2% manganese, 2-5 nickel, 0.2-0.5% nitrogen, OAS-0.80% silicon, and from about 0.1 to about 1.0% niobium, all by weight, the balance being substantially iron.

References Cited by the Examiner UNITED STATES PATENTS 2,225,730 12/1940 Armstrong -l28 3,149,965 9/1964 Jennings 75-128 FOREIGN PATENTS 3.812.814 7/1963 Jana-n.

HYLAND BIZOT, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. 

1. A CORROSION-RESISTANT COMPOSITION OF AUSTENITIC STEEL CONSISTING ESSENTIALLY OF, BY WEIGHT, SUBSTANTIALLY 0.40.8% CARBON, 18-23% CHROMIUM, 7-12% MANGANESE, 2-5% NICKEL, 0.2-0.5% NITROGEN, 0.45-0.80% SILICON, AND FROM ABOUT 0.1 TO ABOUT 1.0% NIOBIUM, THE BALANCE BEING SUBSTANTIALLY IRON. 